Add rollback migration helpers for change_column_type_concurrently

The following migration helpers:

1. change_column_type_concurrently
2. cleanup_concurrent_column_type_change

Have no proper rollback (undo_) counterparts, so their changes can not be rolled back correctly in case of an incident.

Other than the separate issue discussed in #247489 (closed), they more or less work if they are both successfully executed and then rolled back together.

Their usage is explained in Changing Column Types Guide: the provided example does not work correctly in case only the first of the two migrations has to be rolled back by itself: The result of the rollback is going to basically result in a state similar to both the migrations having run, not the initial state of the database.

Given that username is a string column, migrating and then rolling back the following will result to username being a text column.

# A regular migration in db/migrate
class ChangeUsersUsernameStringToText < ActiveRecord::Migration[4.2]
  include Gitlab::Database::MigrationHelpers

  disable_ddl_transaction!

  def up
    change_column_type_concurrently :users, :username, :text
  end

  def down
    cleanup_concurrent_column_type_change :users, :username
  end
end

We should add undo migration helpers for both and update the guidelines to reflect the proper use, similarly to the examples provided for Renaming Columns